How to turn sound notification off?

[Maninogion]

Because I was in a loud environment and was expecting a mail, I went to settings/notification/sounds and chose a ring tone.
Now I want to set the sound notification /sounds to 'None', but am unable to find the setting.
All account 'special' settings are set to 'none', but I still get an acoustic notification.

[Kostya Vasilyev]

It's in settings -> message notifications -> tap the sound preference and there should be a "none" / "silent" etc. option in the list, among all the sounds.